The present invention relates to a polytetrafluoroethylene (PTFE) granular powder, a filler-containing PTFE granular powder and preparation processes thereof. Those granular powders are useful as a molding material, have a high apparent density and low electrostatic charge and give a molded article having a high whiteness.
Hitherto, as a process for preparing a PTFE granular powder, a process for stirring in water containing an organic solvent, a process for stirring in hot water and a process for applying a mechanical force (tumbling) to a powder wetted with a small amount of water are known. Among those processes, as a process for tumbling in a wet state, for example, there is known a process for forming a powder into a slurry with the use of an organic liquid and then tumbling the slurry while heating up near a boiling temperature of the organic liquid (JP-B-45-9071).
However, the above-mentioned processes have problems such that an organic liquid used is inflammable and harmful to human body and further that equipment cost for carrying out the processes becomes high.
On the contrary, as a process for preparing a filler-containing PTFE granular powder, for example, there are a process disclosed in JP-B-44-22619, in which stirring is carried out in water in the presence of an organic liquid, a process disclosed in JP-B-44-22620, in which a PTFE powder and a filler are formed into a slurry with the use of an organic liquid and then tumbling of the slurry is carried out, and the like process. However, the processes disclosed in each of the above-mentioned patent publications have problems such that an organic liquid used is inflammable and harmful to human body and further that equipment cost for carrying out the processes becomes high.
On the other hand, with respect to a process without using an organic liquid, there is, for example, a process disclosed in JP-A-3-259925, in which a PTFE powder is stirred in water in the absence of an organic liquid. In such a. process using water only, since an apparent density of an obtained PTFE granular powder does not become high, post-treatment becomes necessary.
With respect to other process without using an organic liquid, there is, for example, a process disclosed in JP-B-54-17782, in which a PTFE powder is wetted with a specified amount of an aqueous solution containing a volatile nonionic surfactant and then subjected to tumbling. In the process disclosed in that patent publication, an apparent density of a granular powder is low and excellent flowability cannot be obtained. Also since much amount of a nonionic surfactant is used and removal of it is difficult, it remains in a molded article, which results in causing problems such that mechanical properties of the molded article are lowered and that the surfactant is decomposed due to heat in a molding step, thereby causing coloring of the molded article.
Further a process employing an ionic surfactant instead of the above-mentioned nonionic surfactant can be considered. However when a PTFE granular powder obtained by such a process is molded, a residue (for example, a metal salt) of the decomposed ionic surfactant is generated in the sintering step. The residue is difficult to remove, and has been considered to have problems of causing coloring of the molded article and lowering of mechanical properties of the molded article.
Also the PTFE powder is easy to be electrostatically charged, and thus when mixing, stirring and tumbling steps are carried out for granulation, there occurs electrostatic charge of not less than 50 V due to static electricity. Thus electrostatically charged PTFE powder, when molded, adheres to not only a mold die but also a hopper, feeder, etc. due to static electricity, which results in lowering of flowability.
The present inventors have made intensive studies in view of the above-mentioned problems, and as a result, have found that when granulating a PTFE powder or a mixture powder of a PTFE powder and a filler by applying thereto a mechanical force such as tumbling, use of a specific nonionic surfactant can solve the above-mentioned problems.
Namely an object of the present invention is to provide a process for preparing, without necessity of using an organic liquid, a PTFE granular powder which has a high apparent density, a small electrostatic charge and excellent powder flowability and gives a molded article having a high whiteness and being free from lowering of mechanical properties such as elongation and coloring as mentioned above.
The present invention relates to a process for preparing a PTFE granular powder or a filler-containing PTFE granular powder by granulation of a PTFE powder or a mixture powder of a PTFE powder and a filler; characterized in that 100 parts (part by weight, hereinafter the same) of the PTFE powder or the mixture powder is wetted with 30 to 60 parts of an aqueous solution containing a nonionic surfactant having a hydrophobic segment comprising a poly(oxyalkylene) unit having 3 or 4 carbon atoms and a hydrophilic segment comprising a poly(oxyethylene) unit and then a mechanical force is applied to the wetted PTFE powder or the wetted mixture powder.
Further the present invention relates to a PTFE granular powder obtained by the above-mentioned process, which is characterized in that an apparent density of the granular powder is not less than 0.60 g/ml; flowability of the granular powder is not less than 6 times; an average particle size of the particles in the granular powder is from 400 to 1,000 xcexcm; and an electrostatic charge of the granular powder is not more than 50 V.